The present invention relates to a method for the preparation of surface-modified finely divided silica particles capable of forming a stable organosol in any organic solvent and useful as a hard-coating agent on plastic lenses, fabric finishing agent, thickening agent of oils and the like.
There are hitherto known silica powder products dispersible in an organic solvent to form a stable organosol of which the silica particles have alkoxy groups bonded to the silicon atoms on the surface. Such silica particles can be prepared by hydrolyzing an alkoxy silane compound in the presence of an alkaline compound as a catalyst.
The thus produced silica particles having an alkoxylated surface are usually hydrophilic so that they are not always dispersible in a highly hydrophobic organic solvent to cause agglomeration of the silica particles or gelation. Accordingly, the organic solvents utilizable as a dispersion medium of silica organosol are limited to those without high hydrophobicity such as alcohols, e.g., methyl alcohol, isopropyl alcohol, n-butyl alcohol and the like.
Several proposals and attempts have been made in the prior art to prepare an organosol of silica or other inorganic oxides of which the organic solvent as the dispersion medium has a relatively high hydrophobicity. For example, Japanese Patent Kokai No. 63-182204 teaches a method comprising the steps of: (1) hydrolyzing a hydrolyzable compound, such as alkoxides and derivatives thereof, of an inorganic or metallic element, e.g., silicon, titanium, zirconium, aluminum and the like, in a hydrous alcoholic solvent to prepare a suspension of finely divided hydrated oxide particles in the alcoholic solvent; (2) modifying the surface of the particles by admixing the suspension with a coupling agent such as silane compounds, titanate compounds, aluminum compounds and the like having, in a molecule, at least one non-hydrolyzable organic group and at least one hydrolyzable organic group; and (3) replacing the alcoholic solvent with a desired organic solvent having higher hydrophobicity than the alcoholic solvent.
When a tetraalkoxy silane, for example, is used as the coupling agent in the above mentioned surface-modification method of silica particles, a large number of silanol groups are formed on the surface of the silica particles due to the molar ratio of water as the hydrolyzing agent to the tetraalkoxy silane which is usually larger than 6 so that the effect of surface modification cannot be obtained as desired. In addition, this method is applicable only when the finely divided silica particles have an average particle diameter of 0.05 to 5 .mu.m and practically no surface modification cannot be obtained on finer silica particles. In other words, the particle diameter of the silica particles in a silica organosol obtained by this method is relatively large so that such a silica organosol is not always satisfactory when it is used in practical applications as a hard-coating agent on plastic lenses or a thickening agent of oils in respect of the low mechanical strength and transparency or insufficient thickening effect.